INFLUENCE OF THE HYDROGEN GAS CONTENTS OF A GASEOUS FUEL ON COMBUSTION CHARACTERISTICS

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1 INFLUENCE OF HE HYDROGEN GAS CONENS OF A GASEOUS FUEL ON COMBUSION CHARACERISICS Bogdan HORBANIUC*, Gheorghe DUMIRASCU, Andrei DUMENCU GHEORGHE ASACHI ECHNICAL UNIVERSIY, Iaşi, DEPARMEN OF MECHANICAL ENGINEERING AND AUOMOIVE VEHICLES Abstract. Carbon dioxide emissions represent the main environmental issue characteristic to combustion processes. herefore, any effort in this direction is justified by the benefits that result. he paper puts forward a CO abatement technique involving the enrichment in hydrogen gas of a gaseous fuel (natural gas). he original combustion calculus algorithm developed by the authors takes into account dissociation and its output consists of the flue gases temperature and composition in terms of mole fractions and volumes as well. Analysis of output data obtained for different fuel compositions in terms of H percentage shows that low CO flue gases result and thus the main hypothesis is confirmed. Other consequences are also considered and highlighted, confirg that fuel enrichment in H can represent a promising candidate for CO emissions abatement. Keywords: combustion, carbon dioxide emissions, gaseous fuel, hydrogen gas enrichment. Rezumat. Lucrarea abordează problematica reducerii emisiilor de dioxid de carbon rezultate din arderea unui combustibil gazos (gaz natural), prin adăugarea de hidrogen generat din surse regenerabile. Ideea aflată la baza articolului constă în aceea că, scăzând ponderea atomilor de carbon din moleculele combustibilului şi crescând procentajul hidrogenului, generarea de CO va fi diuată. Pentru a putea realiza analiza influenţei conţinutului de hidrogen gazos din combustibil asupra arderii şi a compoziţiei gazelor de ardere, a fost dezvoltat un algoritm original de calcul care ia in considerare disocierea produşilor de ardere. Pe baza acestui algoritm, a fost dezvoltat un pachet software care permite luarea în considerare inclusiv a umidităţii aerului şi a coeficientului de exces de aer. Rezultatele obţinute evidenţiază scăderea procentului de dioxid de carbon din gazele de ardere, concomitent cu creşterea temperaturii produşilor de ardere. De asemenea, se observă o creştere a ponderii monoxidului de carbon datorită disocierii mai pronunţate a CO. Volumul gazelor de ardere scade cu creşterea procentului de hidrogen, efect care, coroborat cu scăderea fracţiei molare a dioxidului de carbon, face ca pe ansamblu, emisiile de CO să scadă. Acest fapt justifică luarea în considerare a acestei metode drept candidat pentru reducerea emisiilor de dioxid de carbon în cazul arderii combustibililor gazoşi. Cuvinte cheie: combustie, emisii de dioxid de carbon, combustibil gazos, îmbogăţire cu hidrogen gazos. 1. INRODUCION Carbon dioxide emissions resulted from industrial combustion processes, especially from power plant boilers, represent a very important environmental issue in the context of the climate change triggered by increased greenhouse gases emissions. A good understanding of the combustion mechanisms and consequently of CO generation allows designing various techniques of CO emissions abatement. By following this path, a number of ideas have arisen among which CO sequestration proved to be one of the most promising. By this technique, the carbon dioxide gas generated in combustion processes is separated from flue gases, purified and subsequently sequestrated in special locations (Russo et al., 13). he resulting * Corresponding author: bogdan_horbaniuc@yahoo.com flue gases are carbon dioxide-free and thus they no longer harm the environment. hree leading capture methods are presently in view (Acharya et al., 5): post-combustion scrubbing, gasification with pre-combustion carbonization, and oxycombustion. Post-combustion scrubbing removes CO by means of a sorbent (i.e., MEA methylethylae) that regenerates by releasing the gas (Radgen et al., 6), (Russo et al., 13). Gasification with precombustion de-carbonization involves gasification of coal into syngas that is subsequently converted into a H -CO mixture from which carbon dioxide is separated, resulting only hydrogen (Acharya et al., 5). Oxycombustion involves fuel combustion with pure O produced by an air separation unit (ASU). his way, the flue gases will only contain water vapor (that can be condensed) and CO that can be easily captured after condensation (Figueroa et al., 8; Metz et al., 5). 18 ERMOEHNICA 1/15

2 INFLUENCE OF HE HYDROGEN GAS CONENS OF A GASEOUS FUEL ON COMBUSION CHARACERISICS A less radical CO abatement technique might be to burn a gaseous fuel (i.e., natural gas) enriched with hydrogen gas produced from renewable sources. he idea behind this solution is to reduce CO emissions and at the same time to take advantage of the heating values of the hydrocarbons composing the gaseous fuel, which are significantly higher than the heating value of hydrogen. In this paper we study the impact of an increasing hydrogen gas content of a gaseous fuel on the adiabatic flame temperature and on the flue gases composition in terms of CO and CO contents.. MAHEMAICAL MODEL OF HE COMBUSION PROCESS.1. Combustion calculus We consider a gaseous fuel the composition of which is: 86.5% CH 4, 7.9% C H 6,.% C 3 H 8,.3% C 4 H 1,.5% CO, and.6% N. By considering the mole fractions of the components expressed in m 3 component/m 3 fuel and symbolized in brackets e.g. for CH 4 : (ch4), we have written the stoichiometric combustion equations, which allowed us to detere the imum volume of oxygen gas necessary for the complete combustion of the unit volume of fuel: O ( 4) 3.5( 6) ( c h ) ( c h ) V = ch c h. () By assug that the combustion air is dry and that the excess air λ = 1, the combustion air volume is: V air O V = = 4.76VO. (3).1 Figure 1 shows the schematic of the combustion chamber with the fuel and air inflows and with the flue gases outflows. he material balance of the combustion chamber is presented in able 1... Dissociation Due to the high temperatures that develop during combustion, some of the resulting species dissociate in more simple molecules and these chemical reactions remove heat from the flue gases, the result being a decrease of the adiabatic flame temperature. We have developed an original model that provides the final flue gases temperature (after dissociation) along with the composition of the combustion effluents. Fig. 1. Schematic of the combustion chamber. Material balance of the combustion chamber able 1 IN REACANS Fuel: (ch4)(ch6)(c3h8)(c4h1) OU PRODUCS Minimum volume of oxygen: combustion VO ch4 35. ch6 5 c3h8 65. c4h1 VHO ch4 3 ch6 4 c3h8 5 c4h1 DO NO PARICIPAE IN COMBUSION REACIONS (CO ) (CO ) V in N V N n fg in V V V n N N N V combustion ch4 ch6 3 c3h8 4 c4h1 CO ERMOEHNICA 1/15 19

3 Bogdan HORBANIUC, Gheorghe DUMIRASCU, Andrei DUMENCU able displays the dissociation reactions and their outcome. he reactions have been divided into primary (dissociations of combustion effluents) and secondary ones (some of the products of primary reactions dissociate themselves or react with other products). Dissociation reactions synopsis able In order to obtain the dissociated fractions, one must detere the equilibrium constant K for each of the dissociation reactions. We have used the general equation: C D zc zd K =. (4) A B za zb where z A, z B, z C and z D are the equilibrium mole fractions of reactants and products of a generic equation A A B B«C C D D. We have supposed a temperature dependence of the equilibrium constants described by the law: 1 3 log K =- x1 ( log) x x3 x4 x5. (5) where the coefficients x 1 x 5 can be detered as the solution of a set of five linear equations of the type (5) for a temperature range comprising five known values of the equilibrium constants corresponding to equally-distributed temperatures in the range. As long as these coefficients have been detered, one can calculate the value of the equilibrium constant for any temperature in the considered range. With this value, the dissociated fractions can be detered by using the specific ERMOEHNICA 1/15

4 INFLUENCE OF HE HYDROGEN GAS CONENS OF A GASEOUS FUEL ON COMBUSION CHARACERISICS forms of Eq. (5) written for each of the dissociation reactions... Flue Gases emperature after Dissociation he flue gases temperature can be detered from the heat balance equation written for the combustion chamber. We have considered that the air-fuel mixture is preheated at a temperature of 59 K ( PH = 59 K). he heat balance equation results as follows (Horbaniuc et al., 1): (, ( )) H H = H V. (6) f a 1g 1g i 1g where: H f fuel enthalpy at the preheat temperature PH ; H a combustion air enthalpy at the preheat temperature PH ; H 1g flue gases enthalpy at the temperature 1g after dissociation, as a function of this unknown temperature and of the temperaturedependent flue gases composition; V i volumes of flue gases components as they result after dissociation he enthalpies that intervene in Eq. (6) can be written as follows: he fuel enthalpy: H f = ( ch4)( hf ) cp, CH d 4 CH 4 ( ch6)( hf ) cp, C d H 6 CH 6 ( ch 3 8)( hf ) cp, C d CH 3H ( c41 h )( hf ) cp, C d CH 4H ( co)( hf ) cp, CO d CO J ( n) cp, Nd ê 3 ëmfuel (7) he combustion air enthalpy: H = V êx c d x c d a a O p, O N p, N ê æ ö J x HO ( h ) f c HO p, HOd ú 3 ç ú êm fuelú çè ë û ø (8) Flue gases enthalpy: H1 g = VCO ( h ) f cp, CO d CO VCO ( hf ) cp, COd CO VHO ( h ) f cpho, d HO VOH ( hf ) cp, OHd OH VO ( hf ) cp, Od VH ( hf ) O cp, Hd H VN ( hf ) cp, Nd N g g VNO ( hf ) cp, NOd VN c p, N d NO g g J VO c p, O d V H c p, H d ê 3 m fuel ú ë û (9) In the above equations, h f accounts for the i enthalpy of formation of component i. he heat capacities have been considered as temperaturedependent and corresponding polynomials from literature have been used (Sonntag&Van Wylen, 198). For the heat capacities of O, H, and N, we took the constant value.973 kj/m 3 K. In order to solve Eq. (6) which is a transcendental one, we have chosen to use the Newton- Raphson technique which is very effective, but it involves the laborious procedure of obtaining the derivative of the function. Once this step is accomplished, the algorithm is rather simple and easy to transpose into computer code. 3. RESULS AND DISCUSSION In order to obtain the required numerical results, we have developed a software that calculates the flue gases temperature and the flue gases composition for different hydrogen gas contents in the natural gas fuel. By entering the ERMOEHNICA 1/15 1

5 Bogdan HORBANIUC, Gheorghe DUMIRASCU, Andrei DUMENCU value of the hydrogen gas percentage, this software generates output screens. Four screenshots are displayed in Fig. for % H, 5% H, 9% H, and 1% H respectively. a. b. c. d. Fig.. Screenshots of results windows for different values of the H percentage in fuel: a %; b 5%; c 9%; d 1%. Fig. 3 shows the dependence of the flue gases temperature after dissociation versus the percentage of hydrogen gas in fuel. he plot exhibits an increase of the temperature as the H percentage increases: at % H fg = K, whereas at 1% H, its value is fg = K. In Fig. 4 we have represented the variation of the carbon dioxide mole fraction in flue gases ERMOEHNICA 1/15

6 INFLUENCE OF HE HYDROGEN GAS CONENS OF A GASEOUS FUEL ON COMBUSION CHARACERISICS versus the H percentage. he range stretches from at 1% H to.814 at % H. One can notice that the higher the hydrogen gas contents in the fuel, the lower the CO mole fraction is in the flue gases. Obviously, with 1% H there are no CO emissions but this is a limit situation; the aim is to reduce carbon dioxide emissions by adding up hydrogen gas to the natural gas fuel and not by totally replacing it by another fuel. Fig. 5 displays the plot CO contents in flue gases versus the H percentage in fuel. he graph shows that as the H percentage increases, so does the carbon monoxide mole fraction. his seegly unexpected outcome can be explained by the fact that the higher the temperature, the more intense is the CO dissociation the result of which is the formation of CO. As shown by Fig. 3, there is a relation between the H percentage and the flue gases temperature in that a higher percentage leads to a higher temperature and thus to a higher CO mole fraction. In Fig. 6 we have plotted the flue gases volume versus the H percentage of the fuel. he graph shows that the dependence is linear, the increase in H contents of the fuel detering a linear decrease of the flue gases volume. In terms of CO emissions, this result, corroborated with the decrease of the CO mole fraction that accompanies the increase of the hydrogen gas contents of the fuel, evidences a double beneficial effect: as the H percentage increases, both the CO mole fraction and the flue gases volume decrease, which means that by adding up hydrogen gas to the fuel, CO emissions decrease in two ways: by reducing the carbon dioxide mole fraction in flue gases and simultaneously by decreasing the flue gases volume. Fig. 3. Plot of the flue gases temperature versus the H percentage in fuel. Fig. 4. he CO mole fraction in flue gases versus the H contents in fuel. ERMOEHNICA 1/15 3

7 Bogdan HORBANIUC, Gheorghe DUMIRASCU, Andrei DUMENCU Fig. 5. Variation of the CO mole fraction in flue gases versus the H percentage in fuel. Fig. 6. he flue gases volume versus the H percentage in fuel. 4. CONCLUSIONS By injecting hydrogen gas in a gaseous fuel in a controlled proportion in the range 1 9%, the flue gases temperature increases and thus the performance of the combustion section of a boiler or of a gas turbine may improve, provided that the parts that enter into contact with the flue gases can withstand higher temperatures. he carbon dioxide emissions decrease with the increase of hydrogen percentage and the flue gases volume diishes concurrently, thus leading to an overall CO abatement. Further investigation will consider using oxycombustion instead of atmospheric air combustion in order to assess the combined influence of the oxidant and of hydrogen gas on combustion characteristics. REFERENCES [1] Acharya D., Krishnamurthy K.R., Leison M., MacAdam S., Sethi V.K., Anheden M., Yan J., Development of a High emperature Oxygen Generation Process and its Application to Oxycombustion Power Plants with Carbon Dioxide Capture, he nd Annual International Pittsburgh Coal Conference, September 11-15, Pittsburgh, Pennsylvania, USA, 5. [] Figueroa J.D., Fout., Plasynski S., McIlvried H., Srivastava R.D., Advances in CO capture technology - he US Department of Energy's Carbon Sequestration Program, International Journal of Greenhouse Gas Control,, 9-, 8. [3] Horbaniuc B., Dumitraşcu Gh., Panaite C.E., NO x emissions of a natural gas-fired steam boiler using oxycombustion, Environmental Engineering and Management Journal, 9, , 1. [4] Metz B., Davidson O., De Coninck H.C., Loos M., Meyer L.A., IPCC Special Report on Carbon Dioxide Capture and Storage: Prepared by Working Group III of the Intergovernmental Panel on Climate Change. IPCC, Cambridge University Press, Cambridge, United Kingdom and New York, USA, 5. [5] Radgen P., Cremer C., Warkentin S., Gerling P., May F., Knopf S., Assessment of echnologies for CO Capture and Storage, Federal Environmental Agency (Umweltbundesamt), Berlin, 6. [6] Russo M.E., Olivieri G., Salatino P., Marzocchella A., CO Capture by Biomimetic Adsorption: Enzyme Mediated CO Absorption for Post-Combustion Carbon Sequestration and Storage Process, Environmental Engineering and Management Journal, 1, , 13. [7] Sonntag E., R., Van Wylen G., Introduction to hermodynamics, Classical and Statistical, Second Edition, John Wiley & Sons, New York, ERMOEHNICA 1/15